Iron surface reconstruction

Adsorbate-induced surface reconstruction and dissociative chemisorption are merely natural extremes of this delicate balance. In each case, strong surface-adsorbate interactions direct the course of the transformation, either breaking up bonding in the surface, so that it reconstructs, or disrupting the adsorbed molecule.59 An incisive discussion of the latter situation for the case of acetylene on iron and vanadium surfaces was provided by A. B. Anderson.60... 

J.A. Dumesic, H. Topspe, and M. Boudart. Surface, Catalytic, and Magnetic Properties of Small Iron Particles. III. Nitrogen Induced Surface Reconstruction. J. Catal. 37 513 (1975). 

It should be stressed that supported iron and cobalt-based catalysts are the only ones applied in industrial plants. In fact the earlier Sasol reactors operated using iron catalysts while the more recent plants utilise cobalt. The historical achievements in the FT reaction have been revised on several occasions see for instance the reviews by Vannice, Schulz and the special issue of Catalysis Todaf devoted to FT Also, a synopsis of the main recent industrial developments has been presented by Adesina. Furthermore, it is worth noting the issue of Topics in Catalysis where different aspects of the reaction mechanism, surface reconstruction of active surfaces, improved reactors and optimisation of catalyst preparations have been treated by various specialists, scientists and engineers. 

According to the model, it can be calculated for the quantitative relationships between the atoms of iron, oxygen and promoters on the sm-face of a model catalyst. Therefore, knowledge of the number of adsorbed oxygen atoms on the iron surface is necessary. The iron faces Fe (100) and Fe (111) are considered in the subsequent calculations. The surface structm-e formed by oxygen atoms on the Fe (100) face is described as Fe (100) p (1 x 1)—O, and the number of oxygen atoms in the monolayer is 12 X For the Fe (111) surface an appropriate reconstruction is... 

In the first reconstruction [27] of road slabs contaminated with CL, silicon iron anodes were embedded in a layer of coke breeze as shown in Fig. 19-4a or the current connection was achieved with noble metal wires in a conducting mineral bedding material. Slots were ground into the concrete surface for this purpose at spacings of about 0.3 m (see Fig. 19-4b). This system is not suitable for vertical structures. 

The physical origin of this structural flexibility of the FeO overlayer is still unclear, the more so since no clear trend is observable in the sequence of lattice parameters of the coincidence structures. The FeO(l 11) phase forming up to coverages of 2-3 ML is clearly stabilized by the interactions with the Pt substrate since FeO is thermodynamically metastable with respect to the higher iron oxides [106,114], FeO has the rock salt structure and the (111) plane yields a polar surface with a high surface energy [115], which requires stabilization by internal reconstruction or external compensation. The structural relaxation observed in the form of the reduced Fe—O... 

In the case of multi-component alloys and compounds, the surface composition may also change in addition to surface relaxation and reconstruction. For instance, the first layer of (100) plane on the surface of a nickel-aluminiim alloy enriches itself with aliuninum whose atomic size is larger than nickel. Such an enrichment of some constituents on the soUd surface is called surface segregation [Van Hove, 1993]. It is abo known that surface active minor impurities of oxygen, phosphorus and sulfur in metallic iron segregate to the clean siirface of iron [Nii-Yoshihara,... 

In a separate series of experiments, the influence of sulfur on the decomposition of a mixture consisting of CO/C2H4/H2 over iron was investigated. Previous work [17] had shown that while iron did not catalyze the decomposition of ethylene, even in the presence of hydrogen, when a small fraction of CO was added to the reactant, a dramatic increase in the rate of decomposition of the olefin was observed. This behavior was rationalized according to a model in which the presence of coadsorbed CO resulted in what is believed to be reconstruction of the iron to form a surface, which favors dissociative chemisorption of ethylene. In the current study, we have extended this study to include the case where sulfur is preadsorbed on the metal surface in an attempt to determine how such adatoms modify the coadsorption characteristics of CO and C2H4 on iron. 

Water adsorbs into the walls of a glass container, but that adsorption is the extent of its adhesion. Some adsorbed molecules react chemically with some types of containers in a process called chemical adsorption (chemisorption. For example, carbon monoxide chemisorbs with palladium, but not with gold). The bonds resulting from chemisorption can hold molecules to the surface with far greater force than would exist with only physical attraction. It is also possible for a molecule (that normally would not chemisorb with the container wall) to break up when hitting the wall s surface. At that point the molecule s constituent parts chemisorb with the container walls. When an adsorbed gas reacts with the materials of a container, it is called reconstruction (for example, the reconstruction of iron with oxygen is rust). 

The necessity of covalent bonding between anions and cations of the reconstructed layer allows May and Carroll ) to discard the possibility of finding protons in such layers. Consequently the presence of hydrogen, as observed in the form of — SH by certain authors on polycrystaUine films of iron ), nickel and tungsten ), may be an indication that the state of the surface is not that of a proper reconstructed layer. 

In closing, it is important to note that the CO/CO2 adsorption technique effectively titrates the active sites for WGS on magnetite catalysts which differ in activity by over an order of magnitude. Nitric oxide on the other hand titrates all of the surface cation sites and is unaffected by Si-substitution. Indeed, NO is known to chemisorb strongly on iron oxides and may even be able to reconstruct the surface. Thus, the combined use of NO and CO/CO2 adsorption provides information about the total magnetite surface area and fraction of the magnetite surface which is active for the WGS reaction. 

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